Unprotected anal sex, regardless of whether it is practiced by straight or gay couples, is considered the riskiest activity for sexually transmitted diseases because of the physical design of the anus: It is narrow, it does not self-lubricate, and the skin is more fragile and likely to tear, allowing STDs such as HIV and hepatitis easy passage into the bloodstream.
To make matters worse, the area is an ideal home for STDs. Bacterial infections such as gonorrhea and chlamydia love warm, moist environments and the type of cells that line the anus.
While unprotected anal sex is much more risky for the receptive partner, the insertive partner is not free from risk. And both partners are susceptible to picking up herpes, syphilis, and HPV even if they use a condom, because sores and warts can reside both inside and outside the anus. In the case of herpes, transmission can occur even in the absence of any genital lesions.
How to reduce risk
Wearing a condom is the best way to reduce the risk of STD transmission. It won't protect you 100% because it won't cover all the areas in which STDs can lurk. And when not worn correctly, condoms can break. But wrap it up correctly every time and you'll drastically reduce your risk. Use plenty of lube, both for the receptive partner's comfort and to reduce the risk of abrasion or small tears to the tissue, which can make STD transmission easier.